This invention relates to a method of producing by chemical vapour deposition (hereinafter referred to as CVD) coloured single crystal diamond, and in one aspect a method of producing fancy coloured diamond, these coloured diamonds being suitable, for example, for preparation for ornamental purposes or applications in which colour is a secondary parameter that may influence market acceptance.
Intrinsic diamond has an indirect band gap of 5.5 eV and is transparent in the visible part of the spectrum. Introducing defects or colour centres, as they will be called, which have associated energy levels within the band gap gives the diamond a characteristic colour which is dependent on the type and concentration of the colour centres. This colour can result from either absorption or photoluminescence or some combination of these two. One example of a common colour centre present in synthetic diamond is nitrogen which, when sitting on a substitutional lattice site in the neutral charge state, has an associated energy level ˜1.7 eV below the conduction band—the resulting absorption gives the diamond a characteristic yellow/brown colour.
Methods of depositing material such as diamond on a substrate by CVD are now well established and have been described extensively in the patent and other literature. Where diamond is being deposited on a substrate, the method generally involves providing a gas mixture which, on dissociation, can provide hydrogen or a halogen (e.g. F,Cl) in atomic form and C or carbon-containing radicals and other reactive species, e.g. CHx, CFx wherein x can be 1 to 4. In addition, oxygen containing sources may be present, as may sources for nitrogen, and for boron. Nitrogen can be introduced in the synthesis plasma in many forms; typically these are N2, NH3, air and N2H4. In many processes inert gases such as helium, neon or argon are also present. Thus, a typical source gas mixture will contain hydrocarbons CxHy wherein x and y can each be 1 to 10 or halocarbons CxHyHalz wherein x and z can each be 1 to 10 and y can be 0 to 10 and optionally one or more of the following: COx, wherein x can be 0.5 to 2, O2, H2, N2, NH3, B2H6 and an inert gas. Each gas may be present in its natural isotopic ratio, or the relative isotopic ratios may be artificially controlled; for example hydrogen may be present as deuterium or tritium, and carbon may be present as 12C or 13C. Dissociation of the source gas mixture is brought about by an energy source such as microwaves, RF (radio frequency) energy, a flame, a hot filament or jet based technique and the reactive gas species so produced are allowed to deposit onto a substrate and form diamond.
CVD diamond may be produced on a variety of substrates. Depending on the nature of the substrate and details of the process chemistry, polycrystalline or single crystal CVD diamond may be produced.
It is well known that post growth treatment such as irradiation with sufficiently energetic particles (electron, neutron etc) to produce lattice defects (interstitials and vacancies) and suitable annealing can result in the formation of colour centres, such as the nitrogen vacancy [N-V] colour centre, which can give the diamond a desirable colour (see for example EP 0 615 954 A1, EP 0 326 856 A1 and the references cited therein). Further characteristics and artificial production of colour centres are discussed in detail by John Walker in the Reports on Progress in Physics, Vol. 42 1979. The artificial production method of colour centres outlined therein comprises the steps of forming lattice defects in crystals by electron beam irradiation and, if necessary annealing to cause the lattice defects to combine with nitrogen atoms contained in the crystals. However, there are limitations to the colours and uniformity that can be produced as a consequence of competitive defect formation and because of the strong sector dependence associated with defects such as nitrogen in diamond.
The colour of a diamond produced by utilising this post growth colour centre formation method is the colour of the rough diamond combined with the colour of the colour centre produced. In order to obtain the ornamental value desired, and thus achieve a combination of high transparency and fancy colour, it has been usual practice to use diamonds that were initially either transparent or light yellow.
There are three visual attributes to colour: hue, lightness and saturation. Hue is the attribute of colour that allows it to be classified as red, green, blue, yellow, black or white, or a hue that is intermediate between adjacent pairs or triplets of these basic hues (Stephen C. Hofer, Collecting and Classifying Coloured Diamonds, 1998, Ashland Press, New York).
White, grey and black objects are differentiated on a lightness scale of light to dark. Lightness is the attribute of colour that is defined by the degree of similarity with a neutral achromatic scale starting with white and progressing through darker levels of grey and ending with black.
Saturation is the attribute of colour that is defined by the degree of difference from an achromatic colour of the same lightness. It is also a descriptive term corresponding to the strength of a colour. The diamond trade uses adjectives such as intense, strong and vivid to denote different degrees of saturation assessed visually. In the CIELAB colour system, saturation is the degree of departure from the neutral colour axis (defined by saturation=[(a*)2+(b*)2]1/2, see hereinafter). Lightness is a visual quality perceived separately from saturation.
The dominant colour of much of the diamond of the invention described hereinafter is brown. Brown is generally a darker, less saturated version of orange. As brown becomes lighter and more saturated it becomes orange. Brown colours also underlie a portion of the yellow hue family so that orange-yellow and orangish yellows in their darker and weaker variants may fall into the brown region.
For diamonds, intermediate colour descriptions between brown and orange are used. In order of decreasing browness and increasing orangeness, the description of the colour goes through the following sequence: brown, orangish brown, orange-brown, brown-orange, brownish orange, orange. Similar sequences apply for the transitions from brown to orange-yellow or orangish yellow. In three-dimensional colour space the region of brown colours is also bordered by pink colour regions and on moving from brown to pink the following sequence is followed: brown, pinkish brown, pink-brown, brown-pink, brownish pink, pink.
Fancy coloured diamonds are diamonds with an obvious and unusual colour. When the dominant component of that colour is brown they are described as fancy brown. This term covers a complex range of colours, defined by a three dimensional region of colour space. It covers large ranges in the values of lightness, hue and saturation.
The inherent colour of a cut diamond, sometimes called the body colour, can best be judged if the diamond is viewed from the side for typical cuts. The apparent colour seen in the face-up direction (ie looking towards the table) can be greatly affected by the cut of the stone because of the effect that this has on the path length within the stone for the light subsequently reaching the eye. For example, inherently orange-brown diamond can be cut in such a way that its face-up colour appears brighter, resulting in a reversal of the dominant colour to brown-orange.